Most subjects with heart disease are normolipemic. Little is known about lipoprotein-mediated cellular mechanisms of atherogenicity in normal subjects or even in hypercholesterolemic subjects, since native fasting lipoproteins from such subjects do not appear to be atherogenic in vitro [do not cause lipid accumulation in human monocyte-macrophages (HMM)]. Humans are often in a postprandial state and the idea that atherosclerosis is a postprandial phenomenon is widely accepted. Little is known about the interactions of specific postprandial triglyceride-rich lipoprotein subspecies (ppTGRLP) with HMM, however, interactions that are likely to contribute to atherogenesis. The overall goal of this proposal is to determine cellular mechanisms of atherogenicity of ppTGRLP subspecies that appear transiently during the postprandial response in certain normal and hyperlipoproteinemic subjects at increased risk for atherosclerosis (low HDL levels; small, dense LDL pattern; elevated LDL; hyperapoB; higher fasting TG levels; maleness; apo E4 phenotype). Metabolically stabilized subjects will consume a meal containing vitamin A to label intestinally-derived particles; TGRLP will be isolated before and at specific times postprandially, subfractionated by cumulative flotation into homogeneously sized subclasses. Fasting and ppTGRLP subclasses will be tested for rapid cholesteryl ester and triglyceride loading of human THP-1 monocyte-macrophages as in vitro tests for potential atherogenicity; selected atherogenic ppTGRLP will be subfractionated by immunoaffinity chromatography into apoB-100 vs B-48 particles to determine the origin of atherogenic species. Mechanisms of uptake will be determined (via the LDL receptor, the distinct macrophage receptor for abnormal TGRLP, or other mechanisms). Structural determinants of functional abnormalities of each atherogenic ppTGRLP subclass will be identified, tracked with time postprandially, and correlated with known risk factors. These studies should identify basic cellular mechanisms involved in human atherosclerosis (conversion of macrophages into foam cells) and factors responsible for the occurrence of atherogenic ppTGRLP. Knowledge of the basic biochemical determinants of potentially atherogenic ppTGRLP/cellular interactions may lead to development of rational dietary and therapeutic measures to minimize the expression of atherogenic ppTGRLP and, possibly, to better means of determining risk in otherwise normolipemic subjects.